A normal DLL can export any function. 
 A COM DLL is the one which exports COM APIs like
            * DllGetClassObject.
            * DllRegisterServer.
            * DllUnregisterServer.
            * DllCanUnloadNow.

COM Dll exposes Interface on the contrary to normal DLL 
that exports functions. Clients create the pointer to 
COMDLL's interface to call the methods defined by the 
component that implements the interface. 
That results in isolation of implementation and definition 
of method in the interface. Client doesnt need to relink or 
recompile the code if method in the com dll changes as far 
as the definition of the interface remains same.

Classic DLLs are function libraries while ActiveX(COM)
DLLs are objects. The difference is like the difference
between procedural and object oriented programming.
     API style Declare statements are needed for routines in
a 'normal' DLL and and an ActiveX(COM) DLL is treated like a
class object.